1. Field of the Invention
This invention relates zoom lenses and more particularly to a zoom lens comprising, from front to rear, a first component for focusing, a second component of negative power that performs a image magnification varying function, a third component of negative power that serves for image shift compensation and a fourth component of positive power for forming an image.
2. Description of the Prior Art
State of the art of video cameras have been the subjects of great improvements in image sensing and electrical circuit techniques, and presently available image pick-up tubes have excellent image response characteristics in areas such as resolving power and color balance. Such advances create a demand for a zoom lens of reduced size, light weight and high relative aperture with a large zoom ratio usable with many video cameras. Also, single-tube color cameras of the phase separation type and the three-electrode type require that the optical system be constructed in telecentric form for the purpose of preventing color deviation.
In general, most users desire zoom lenses of small size, light weight and short physical length. An effective measure for shortening the mechanical length is to reduce the total movement of the variator.
It is possible to shorten a four component type of zoom lens with a first component counting from the front for focusing the lens, a second component for variation of the image magnification, a third component for image shift compensation and a fourth component for image formation. Such longitudinal shortening of the lens system is advantageously achieved by increasing the refractive power of the second component with decrease in the total movement thereof.
The increase in the refractive power of the second component, however, calls for strengthening the curvature of each refractive surface of the lens elements constituting the second component. This increase the amount of aberrations produced. Under such circumstances, it is very difficult to stabilize good correction of spherical aberration and marginal coma and field curvature throughout the entire zooming range. Further, in the design of a relay lens system with a limitation in the number of lens elements, it is also very difficult to realize a significant increase in relative aperture and form a telecentric optical system.
A zoom lens suited particularly for video cameras and constructed from four components, or a focusing first component, a magnification varying second component, an image shift compensating third component and an image forming fourth component is disclosed in Japanese Laid-Open Patent Application No. SHO 57-19709 of Feb. 2, 1982.
This conventional zoom lens comprises, from front to rear, a focusing first component of positive power with a focal length f1, a magnification varying second component of negative power with a focal length f2 and axially movable to contribute mainly to a change in the focal length of the entire system, a compensating third component of negative power with a focal length f3 and axially movable to maintain the constant position of the image plane which would be otherwise shifted by the movement of the variator, and a relay lens system that follows said compensator. The relay lens system includes a front part or fourth component of positive power with a focal length f4 and a rear part or fifth component having a focal length f5. The fourth component consists of three convex lens members of which at least the central convex lens member is a doublet with its cemented surface convex toward the rear, and the fifth component consists of one or two convex lens member or members, at least one of which is a doublet with its cemented surface convex towards the front. The relay lens system satisfies the following conditions: ##EQU1## where R.sub.A is the radius of curvature of the cemented surface of the central or doublet lens in the fourth component; .nu..sub.AP and .nu..sub.AN are the Abbe numbers of a positive and a negative lens constituting said doublet; R.sub.B is the radius of curvature of the cemented surface of the doublet lens in said fifth component; .nu..sub.BP and .nu..sub.BN are the Abbe numbers of a positive and a negative lens constituting said doublet respectively; F.sub.W is the shortest focal length of the entire lens system; f5 is the focal length of said fifth component; D is the interval between the principal planes of said fourth and said fifth components; Rc is the radius of curvature of the front surface of the positive lens in said fifth component; and Bf is the back focus.
Though the characteristic feature of that zoom lens resides in cemented lenses of the relay optical system, it has been found that the use of the cemented lenses provides unsatisfactory results in the image aberrations and particularly imaging features in terms of field curvature.